CS2422 Assembly Language

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CS2422 Assembly Language System Programming

• September 19, 2006

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Todays Topics (1/2)

• How the CPU executes a program.
• Where we can store the data.
• Registers
• Memory
• Constants

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Todays Topics (2/2)

• Data representation What are stored in the main
  memory?
• Binary, Decimal, Hexadecimaletc.
• Units Byte, Word, Double-wordetc.
• Memory Addresses.
• Characters Strings.
• Big or Little Endian.

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Study Guide

• Textbook Sections 1.3, 2.1 (except 2.1.2.2),
  2.2, 2.3.
• Read (in depth)
• 2.1.1 and 2.1.2.1
• 2.2.1 and 2.2.2
• 2.3.1 and 2.3.2.1
• Browse the rest (like reading news stories).

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Program Loading Execution

• A simpler view
• clock synchronizes CPU operations
• control unit (CU) coordinates sequence of
  execution steps
• ALU performs arithmetic and bitwise processing

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Instruction Execution Cycle

• Fetch
• Decode
• Fetch operands
• Execute
• Store output

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Clock

• Not the wall clock.
• Ever wondering what the CPU clock rate (like
  2.0GHz) means?
• Next slide using the memory read as an example

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Figure 2.7 (from Textbook)
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Cache Memory

• High-speed expensive static RAM both inside and
  outside the CPU.
• Level-1 cache inside the CPU
• Level-2 cache outside the CPU
• Cache hit when data to be read is already in
  cache memory
• Cache miss when data to be read is not in cache
  memory.

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How a Program Runs
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Multitasking

• OS can run multiple programs at the same time.
• Multiple threads of execution within the same
  program.
• Scheduler utility assigns a given amount of CPU
  time to each running program.
• Rapid switching of tasks
• gives illusion that all programs are running at
  once
• the processor must support task switching.

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Data Representations

• Consider this simple view The whole program is
  stored in main memory.
• Including program instructions (code) and data.
• CPU loads the instructions and data from memory
  for execution.
• Dont worry about the disk for now.

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Where are the Data?

• Registers (inside the CPU)
• Memory
• Constant

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Registers

• General-Purpose
• AX, BX, CX, DX 16 bits
• Splitted into H and L parts, 8 bits each.
• Extended into E?X to become 32-bit register
  (i.e., EAX, EBX,etc.).

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Convention

• AX accumulator
• BX base register
• CX count register
• DX data register
• Some instructions use them implicitly. (e.g.,
  LOOP uses CX with no mentioning.)

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Other Registers

• We will explain their meaning when we encounter
  them later this semester
• Segment (CS, DS, SS, ES)
• Pointer (IP, SP, BP)
• Index (SI, DI)
• Flags

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Memory

• Organized like mailboxes, numbered 0, 1, 2, 3,,
  2n-1.
• Each box can hold 8 bits (1 byte)
• So it is called byte-addressing.

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Byte? Word?

• The number has limited range.
• 1 Byte 8 Bits
• Binary 0000 0000 to 1111 1111
• Hexadecimal 00 to FF
• Decimal 0 to 255
• Word 2 or 4 bytes, depending on the machine.
  In 80x86, it means 2 bytes.

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Number Systems

• I assume that now you all know
• Binary hexadecimal numbers.
• Conversion between them and decimal.
• How to represent negative numbers (in 2s
  compliment).

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Memory Address

• Byte Addressing each memory location has 8 bits.
• If we have only 16 bytes
• 4 bits are enough for an address
• What if we have 64K?
• 1M? 1G?

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Memory Address

• 16-bit address is enough for up to 64K
• 20-bit for 1M
• 32-bit for 4G
• Most servers need more than 4G!! Thats why we
  need 64-bit CPUs like Alpha (DEC/Compaq/HP) or
  Merced (Intel).

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Confused Now?

• So, is the memory 8-bit or 32-bit?
• It depends on what you ask
• Content or Address?
• Remember addresses can be stored in memory as
  well. (They are called pointers in PASCAL and C.)

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• Break

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Character String

• So how are strings like Hello, World! are
  stored in memory?
• ASCII Code! (or Unicodeetc.)
• Each character is stored as a byte.
• Review how is 1234 stored in memory?

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Integer

• A byte can hold an integer number
• between 0 and 255 (unsigned) or
• between 128 and 127 (2s compliment)
• How to store a bigger number?
• Review how is 1234 stored in memory?

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Big or Little Endian?

• Example 1234 is stored in 2 bytes.
• 100-1101-0010 in binary
• 04 D2 in hexadecimal
• Do you store 04 or D2 first?
• Big Endian 04 first.
• Little Endian D2 first.? Intels choice

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• Reason for Little-Endian?
• More consistent for variable length (e.g., 2
  bytes, 4 bytes, 8 bytesetc.)

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Writing the First Program

• MOV AX, var1
• MOV BX, 1
• ADD AX, BX
• MOV var1, AX

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Constant (or Immediate) Data

• Compare
• MOV AX, 25
• MOV AX, var1 assume var1 at address 25
• The constant 25 in the first case is called
  immediate data.
• The second case may be considered as
• MOV AX, 25

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Food for Thought

• Example MOV AX, 25
• But, where is the number stored? In memory?
• A related question where is the instruction (MOV
  AX, 25) stored?
• What do we do if we want to move the data in
  memory address 25 to AX?

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Multi-Stage Pipeline

• Pipelining makes it possible for processor to
  execute instructions in parallel
• Instruction execution divided into discrete stages

Example of a non-pipelined processor. Many wasted
cycles.
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Pipelined Execution

• More efficient use of cycles, greater throughput
  of instructions

For k states and n instructions, the number of
required cycles is k (n 1)
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Instruction Set Architecture

• ... the attributes of a computing system as
  seen by the programmer, i.e. the conceptual
  structure and functional behavior, as distinct
  from the organization of the data flows and
  controls, the logic design, and the physical
  implementation.
• Amdahl, Blaaw, and Brooks, 1964
• interface between hardware and low-level software
• standardizes instructions, machine language bit
  patterns, etc.
• advantage different implementations of the same
  architecture
• disadvantage sometimes prevents using new
  innovations

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RISC vs. CISC

• RISC PowerPC, MIPS, DEC Alpha
• CISC x86/Pentium, AMD, VAX
• Example

x86 (CISC) MOV AX, a ADD AX, b
RISC Load R1, a Load R2, b Add R2, R1